1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to signal processing in coherent optical transceivers.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A coherent optical-detection scheme is capable of detecting not only the amplitude of an optical signal, but also the signal's phase. These capabilities make coherent optical detection compatible with the use of spectrally efficient modulation formats, such as quadrature-amplitude modulation (QAM) and phase-shift keying (PSK) in their various forms. Compared to non-coherent optical detectors, coherent optical detectors offer relatively easy wavelength tunability, good rejection of interference from adjacent channels in wavelength-division-multiplexing (WDM) systems, linear transformation of the electromagnetic field into an electrical signal for effective application of modern digital-signal-processing techniques, and an opportunity to use polarization-division multiplexing (PDM).
A coherent optical receiver usually employs an optical mixer that combines a received optical communication signal and a local-oscillator (LO) signal to generate one or more in-phase (I) signals and one or more quadrature-phase (Q) signals. Ideally, the I and Q signals corresponding to the same component of the optical communication signal are orthogonal to one another. However, implementation imperfections, such as incorrect biasing of the optical mixer, imperfect signal splitting in optical couplers, variability in the optical-to-electrical (O/E) conversion characteristics of photodiodes, etc., can create amplitude and phase imbalances between the I and Q sub-channels of an I/Q channel. These imbalances translate into a bit-error-rate penalty, which rapidly grows with an increase in the constellation size, e.g., to a size larger than four constellation points, such as a constellation larger than the quadrature-phase shift keying (QPSK) constellation which has four constellation points.